<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    .outer {
      width: 100%;
      height: 100vh;
    }

    .side {
      width: 300px;
      height: 100%;
      background-color: orange;
    }

    .mid {
      height: 100%;
      margin: 0 300px;
      background-color: lightblue;
    }

    .left {
      float: left;
    }

    .right {
      float: right;
    }
  </style>
</head>

<body>
  <div class="outer">
    <div class="side left"></div>
    <div class="side right"></div>
    <div class="mid"></div>

  </div>
</body>

</html>